Optimization of the Rotor Geometry of a Permanent Magnet Synchronous Machine

Converting an induction machine (IM) to a permanent magnet synchronous motor (PMSM) can be a solution to increase the efficiency. Therefore we started from an original 1.5 kW 6-pole IM and converted it into a 6-pole PMSM. The stator of the 6-pole IM was kept unchanged and the rotor was converted into a permanent NdFeB magnet rotor. Furthermore we want to optimize the rotor geometry to obtain high efficiency and low magnet volume to reduce the magnet cost. In addition the cogging torque should be low, and the mechanical power should be at least equal to the nominal power of the induction machine at nominal current. The optimized parameters are the magnet thickness (tm), the number of magnet segments per pole (Np) and the magnet pole angle ("alpha" m). For simulating the 6-pole PMSM’s a transient 2D finite element model (FEM) was used, taking into account iron and copper losses. A geometry (Np = 5,
"alpha"m = 150°, tm = 3 mm) was found so that the PMSM has 82.7% average efficiency and nevertheless rather low cogging torque (0.51 Nm) and magnet volume (11.1 cm³).